You can measure the burden voltage, pump it in to a formula and you can tell what the output is supposed to be.So why don't ammeters do that and adjust the output number?

I think it is not actually related to the measured number, but to the output voltage supplied to the device you're testing. It may seem like nothing, but try this with something powered by a LDO 3.3 V or even 2 V regulator at marginal input voltage.

It's name comes from the fact that it is a burden on the circuit that is supplying the voltage. Whatever the burden voltage is it is an additional voltage drop that was not in the original circuit before measuring the current. Therefore the circuit is disturbed by the act of measurement and that may change the way the circuit functions. This becomes more critical with very low system voltages.

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Assume the DUT is a diode, how would you correct for burden voltage? Forward current through a diode can be approximated with an exponential function of the forward voltage (Shockley's diode equation), which depends on several parameters that vary with temperature and device. If the DUT is a switching power supply, then the voltage drop of the burden voltage might cause the current to go up. How is the ammeter supposed to know whether the DUT is a resistor, a diode, or a more complex device?

There is also the problem of showing a hypothetical situation as opposed to showing the current reality. If the ammeter increases the current to compensate for burden voltage, then other concurrent measurements like voltage and heating, will be off.

This is similar to the two competing strategies that Agilent and Tektronix used to have (not sure if this is still current) for probe design. One of them would calibrate a probe so the amplitude from a well defined source impedance as measured by the probe was equal to the unloaded amplitude. The other would calibrate it to the amplitude of the signal with the probe applied. The former strategy has the advantage that it will correct for the probe loading as long as the source is a well behaved 50 ohm source, but falls apart once you apply multiple probes to the same test point. The probe response will correspond to neither the unloaded response (since it's being attenuated by the loading from the second probe), nor the response of the source loaded by two probes. See this appnote (under oscilloscope probing philosophy) for more information.

I was told that A.C. was an option on that meter; it has the switch, but it is locked and can't be moved.Larry

I was not aware of that.If it is the same problem with my device, I shall use it only for DC then.No way to unlock it really?You mean the hardware is there but there is a physical lock preventing the use of it?

Burden voltage is because current is measured by voltage drop across a known low value resistor. The Keithly does this instead by an active measurement of using an opamp virtual earth to generate a voltage proportional to the current internally to keep the input voltage close to zero. TO do this for higher currents than the 1mA offered means you need an opamp capable of sinking or sourcing the current on it's output thus you will need one capable of doing 1A linearly for a 1A measurement, 10A for a 10A unit. This current has to come from a power supply, so will make it both a very big and very expensive unit, especially if you are looking for high accuracy, as high current power amplifiers normally do not have the best offset and drift characteristics at low currents, and definitely not at full power. not much good having a 10mV burden voltage if the opamp has 100mv of drift over a period of 1 minute, or 200mV of offset as it goes from 0 to 10A.

It is a delay fuse, where the resistor is there to go open circuit at massive overload as it is a film unit, but where with moderate overload it gradually heats up till the low melting point solder that is used to hold the one end to a spring ( a tin Gadolinium alloy IIRC) softens and the spring breaks the circuit. It was common as a time delay fuse. A 100% load would be fine. 200% would trip in about 5 hours and 400% would trip in about a minute, while a short ( more than 20A) would blow the resistor element in under a second. It does have significant drop at low current ratings, all values of this construction drop around 5V across them, but they are perfect for mains transformers where they limit inrush current and can be rated at the full load draw of the transformer with little risk of them failing randomly. Normally they were available in ratings up to about 630mA ( biggest I have seen) but came in as low as 15mA ( seen that value) and possibly lower.

The strange cable mounting is there so you can nicely wind the cable around the case stands. I don't like equipment with permanent mains or any other cables sticking out of it, and I carried a lot of it at work. Really annoying unless you have a third hand to hold the cables, otherwise it is easy to trip over the cable.

It's on the bottom because the front and the rear cards on these instrument cases are removable.

I was told that A.C. was an option on that meter; it has the switch, but it is locked and can't be moved.Larry

I was not aware of that.If it is the same problem with my device, I shall use it only for DC then.No way to unlock it really?You mean the hardware is there but there is a physical lock preventing the use of it?

I do have the manual w/ schematics at work someplace, so I can confirm whether or not A.C. is indeed an option (and possibly a module) and not just a stuck or "locked" switch. In that case, the engineer before me didn't know what he was talking about. When I had it open to replace the caps, I didn't take notice what prevented the switch from operating, as I only use the meter for D.C. and resistance. I really don't use that meter that much, but it works quite well...

I was told that A.C. was an option on that meter; it has the switch, but it is locked and can't be moved.Larry

I was not aware of that.If it is the same problem with my device, I shall use it only for DC then.No way to unlock it really?You mean the hardware is there but there is a physical lock preventing the use of it?

I do have the manual w/ schematics at work someplace, so I can confirm whether or not A.C. is indeed an option (and possibly a module) and not just a stuck or "locked" switch. In that case, the engineer before me didn't know what he was talking about. When I had it open to replace the caps, I didn't take notice what prevented the switch from operating, as I only use the meter for D.C. and resistance. I really don't use that meter that much, but it works quite well...

I am just reading the manual right now. It looks like it does not measure currents AC or DC!hahaThat's sounds great;)Only AC and DC voltages and resistances can be measured. There is no current button on the front panel anyway.Next time I shall read the manual before buying.The only Option they mention is the data output option.May be that engineer was talking about the current measurement, right?

The 8921A is 4 1/2 digits, has up to 20 MHz bandwidth, 700V max input, and resolution down to 0.1 1 uV on lowest range, and also selectable dB scale. Great stuff! Very useful for measuring residual noise on (switching) power supplies for instance.

CarlG, the manual I looked at showed 180µV as the lower functional limit independent of resolution. Is that what you find in practice. If you use it below 180µV how low does it go and what did you use to verify its accuracy below the 180µV. I am not disputing your info, I just may be interested in one if it works well in the single digit µV.

The 8921A is 4 1/2 digits, has up to 20 MHz bandwidth, 700V max input, and resolution down to 0.1 1 uV on lowest range, and also selectable dB scale. Great stuff! Very useful for measuring residual noise on (switching) power supplies for instance.

CarlG, the manual I looked at showed 180µV as the lower functional limit independent of resolution. Is that what you find in practice. If you use it below 180µV how low does it go and what did you use to verify its accuracy below the 180µV. I am not disputing your info, I just may be interested in one if it works well in the single digit µV.

I'll have to check when I get back to work on Monday, but as I remember it, the noise floor is better than that. On the other hand, my memory has a tendency to be a bit optimistic in some situations

Note that I'm only speaking of resolution, not accuracy at that level.

Ok. I haven't read all of the 8921a manual, but I see that the underload condition is used by the Auto ranging function to step to the next range. I don't see it (underload cond) directly related to the accuracy?

The mysterious empty DIP connector near the big IC seems to be another diagnostic connector (check J1002 on the right size of diagram) with basically all the digital communication with the display board available on its pins.

Nope. That's the tap into the BCD data coming from the A/D convertor. This goes to the optional GPIB board. what they do is simply catch the digit BCD data and digit select signals. this gets stored in a block of circuitry so it can be read over gpib.

Looks like an interesting machine. just found one on ebay for 69$ including the gpib board... on its way now ...

« Last Edit: January 03, 2013, 06:32:32 pm by free_electron »

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Ok. I haven't read all of the 8921a manual, but I see that the underload condition is used by the Auto ranging function to step to the next range. I don't see it (underload cond) directly related to the accuracy?

Table 1 specifications shows the measurement ranges are all multiples of 0.18 thru 1.999 except the 700V top end.